What are the mechanisms underlying orientation selectivity? While orientation perception is one of the most extensively studied elements in vision science, much remains unknown about this basic feature of our visual system. The proposed studies couple psychophysics with emerging techniques in cognitive neuroscience to investigate three main questions: (1) What is the source of orientation selectivity? Although orientation processing remains one of the most extensively studied features of the visual system, considerable controversy remains regarding the basic mechanisms underlying this selectivity. In the first experiment, I will use Transcranial Magnetic Stimulation (TMS), a noninvasive tool for temporarily depressing a region of cortical activity, to delimit the source of orientation selectivity in humans. (2) Can orientation selectivity be modulated by states of awareness? In the second experiment, I propose exploring the flexibility of orientation tuning, asking whether an observers'ability to see a stimulus is dictated by our selectivity for that stimulus. I will use binocular rivalry to manipulate the visibility of a stimulus, while psychophysically measuring the bandwidth of orientation selectivity. These results will not only shed light on mechanisms of orientation tuning, but will also reveal a hitherto unexplored aspect of binocular rivalry: the role of selectivity on perceptual states of rivalry. (3) What is the interplay between orientations? In the third experiment, I will investigate the source of an established perceptual illusion: the tilt repulsion effect. It has been suggested that this illusion, which arises from an interaction between different orientation representations, has intracortical origins. I will use TMS, in conjunction with psychophysics, to help isolate the source of this interaction. Upon completion of my training, I plan to pursue a university professorship position researching visual perception, with the goal of linking physiological findings with human behavior. To that end, the broad objective of the proposed studies is to gain expertise with innovative new theoretical and empirical approaches, such as models of orientation selectivity and TMS, allowing me to better establish that link. In addition to improving our understanding of the human visual system, the proposed research stands to provide significant benefits to public health. Studying binocular rivalry is directly relevant to our understanding of visual disorders such as amblyopia, where disparate signals enter the two eyes. Moreover, the proposed studies using TMS will bolster our understanding of a nascent technique, which recently has been FDA approved for treatment of depression, with potential for additional health benefits.